<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{padding: 0;margin: 0;}
        .box{
            width: 200px;
            height: 200px;
            background-color: #6ff;
            margin:100px;
        }
    </style>
</head>
<body>
    <?php
        echo 111111;
    ?>
    <div class="box"></div>
    <?php
        echo 222222;
    ?>
</body>
<script>
    // PHP 代码在服务器上执行，结果以纯 文本/HTML/json 形式返回给浏览器
    var box = document.getElementsByClassName("box")[0];

    box.onclick = function(){
        alert(1111111);
    }

</script>
</html>